Conventionally, a sealed secondary battery used widely for home electric appliances and the like. Especially, a lithium-ion secondary battery is actually used for an electric vehicle because of its high energy density.
When the secondary battery mentioned above is charged with electricity further than the state at which the battery is charged enough (full charge), overcharge occurs, whereby an internal short circuit may be caused between positive and negative electrodes so as to generate gas by resolution of an electrolyte. As a result, the internal pressure of the secondary battery rises, thereby causing exothermicity and failure of the secondary battery.
In the case in which such secondary batteries are employed as an assembled battery, by providing a control circuit in each of the batteries and watching the batteries totally, dispersion of capacities of the batteries and change of degradation speed can be tackled so as to prevent the overcharge. However, providing the control circuit in each of the batteries is difficult because it increases the cost and increases the weight and space.
On the other hand, as a countermeasure for the overcharge of the secondary battery, an art using a current cut-off device of so-called internal pressure reaction type is actually employed in which an electric connection section is disconnected when the internal pressure of the secondary battery becomes not less than a fixed value. Concretely, an art concerning the current cut-off device is well known in which an electrically conductive diaphragm (metal foil) is provided in a lid or a battery casing of the secondary battery, and when the internal pressure of the secondary battery reaches the reversal pressure of the diaphragm, the diaphragm is reversed oppositely to a current collecting plate, whereby the connection between the diaphragm and the current collecting plate is disconnected and the current is cut off (for example, see JP 2007-87730 A and JP 2007-227283 A).
PTL 1: JP 2007-87730 A
PTL 2: JP 2007-227283 A